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- Twinning, common in many animals, is of two biological kinds: the one-egg (monozygotic), or identical, type and the two-egg (dizygotic), or fraternal, type. The latter type is more usual and can be thought of simply as a litter of two.
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Identical twins originate from a single fertilized egg. That egg usually splits in two in the first week of pregnancy and goes on to become two twins with identical DNA. Fraternal twins have different sets of DNA. That’s because they come from separate eggs that were fertilized by separate sperm. Image by Trlkly via Wikimedia Commons.
- Mirror Twins
- Conjoined Twins
- Parasitic Twins
- Semi-Identical Twins
- Female and Male Identical Twins
Mirror twinsare exactly what they sound like! These twins are actual mirror images of each other. This means that: 1. Their hair may naturally fall in opposite directions. 2. Their teeth may grow in on opposite sides of their mouths. 3. They may have birthmarks on the opposite side of their bodies. 4. They also usually have different dominant hands...
Conjoined twins are a rare twin type in which the two siblings are physically connected. Typically, conjoined twins are joined together at the chest or abdomen, but this varies. Some conjoined twins are connected to a greater extent than others. Most share at least one vital organ. Though physically attached to each other, conjoined twins are two i...
Sometimes, as twins develop in utero, one twin becomes larger and more dominant. The other twin stops developing and begins to depend on their sibling. Known as parasitic twins, these twins are physically conjoined. However, the smaller twin is not fully formed and isn’t capable of surviving on their own. This is because the smaller twin is usually...
There have only been two reported cases of semi-identical twins, so this type is extremely rare. In semi-identical twins, two separate sperm fertilize one egg. The fertilized egg then splits in two. So, semi-identical twins share all the same chromosomes from their birthing parent, but only about 50 percent from their non-birthing parent.
Sometimes identical twins can be assigned the sex of male and female at birth. These twins start off as identical males with XY sex chromosomes. But shortly after the egg divides, a genetic mutation called Turner syndromeoccurs, leaving one twin with the chromosomes X0. This twin will be assigned female at birth but may have developmental issues, a...
Mar 7, 2021 · In these animals, two babies born at the same time are called twins, not littermates. However, these are usually fraternal, not identical twins. Sheep , goats , and deer all regularly give...
May 9, 2024 · Twin, either of two young who are simultaneously born from one mother. Twinning, common in many animals, is of two biological kinds: the one-egg (monozygotic), or identical, type and the two-egg (dizygotic), or fraternal, type. The latter type is more usual and can be thought of simply as a litter.
- The Editors of Encyclopaedia Britannica
Jan 1, 2023 · Twins that look exactly alike are typically referred to as "identical" twins. However, a more accurate term is "monozygotic." Monozygotic twins share almost exactly the same genes as they are formed from the same embryo. Learn more about monozygotic twins, including how they are formed and other interesting twin facts.
Fraternal and identical are the most common types of twins. Other types include conjoined twins, half-identical twins and others. Fraternal twins come from two eggs and two sperm.
Identical twins: Monozygotic: x: x: x: x: x: x Fraternal twins: Dizygotic: x: x: x: x: x: x Half-identical twins: Sesquizygotic: x: x: x: x: x: x Mirror image twins: x: x: x: x: x: x: x Mixed twins: x: x: x: x: x: x: x Superfecundation: x: x: Eggs are fertilized during different acts of intercourse: x: x: x
